GABAergic interneurons are crucial in the spatial-temporal regulation of neuronal firing and ensemble activity in cortical circuits. Chandelier cells (ChCs) are a unique class of interneuron with striking features in their innervation: they specifically inhibit pyramidal neurons and specifically at axon initial segments (AIS) - the site o action potential generation. Further, a single ChC innervates hundreds of pyramidal cells in cortex. Therefore, ChCs are poised to powerfully control pyramidal cell firing and ensembles that support motor and cognitive functions. Knowledge on the source and pattern of inputs to ChCs is the basis for understanding how they are recruited and regulated to exert powerful control over pyramidal cell networks under relevant physiological and behavioral contexts. However, the location and identity of presynaptic cells that control ChCs activity is almost entirely unknown. Altered GABAergic markers in prefrontal cortex ChCs is one of the most consistent pathology in post mortem brain of schizophrenic patients. These changes are hypothesized to result from reduced inputs (e.g. local excitatory drive or modulatory pathway, but no direct evidence has been provided. My proposal aims will provide the first set of data on inputs to ChCs and can be applied to mouse models of mental disorders. As such, my proposal will attempt to label the local presynaptic inputs to layer 2 (L2) and L5 ChCs in the medial prefrontal cortex (mPFC). Preliminary data from our lab identified at least two subtypes of ChCs confined to L2 and L5. Additionally, I will label the long range subcortical and contralateral inputs to L2 and L5 ChCs. Finally, in order to assess whether this presynaptic connectivity is conserved across brain regions, I will label the local and long range inputs to L2 and L5 ChCs in the somatosensory cortex to compare with the data from mPFC. To accomplish these aims, I will employ a pseudotyped monosynaptic rabies tracer that was shown to isolate the first order inputs in a cell type specific manner based on Cre expression. [To validate the functional connectivity of inputs to ChCs, Channel Rhodopsin Assisted Circuit Mapping (CRACM) will be used to map the long range presynaptic cells in areas such the contralateral cortex, thalamus and neurmodulatory nuclei.] Preliminary data has established rabies tracing in a Parvalbumin-ires- Cre mouse line, and an inducible Cre line under the promoter of transcription factor Nkx2.1. We have found that induction of Nkx2.1-CreER at a late embryonic stage was found to label ChCs. The project will provide the first mapping of input on ChCs. Because ChCs are implicated in mental disorders such as schizophrenia, their connectivity and function could shed light into the pathophysiology and suggest new treatment strategies. PUBLIC HEALTH RELEVANCE: Mental health disorders such as schizophrenia, bipolar disorder and autism are currently the leading form of disability and contribute greatly to total health care costs in the United States with 1 in 4 Americans suffering from mental disorder. Chandelier Cells are types of powerful inhibitory neurons in the brain that are altered in schizophrenia. This research will lay the groundwork to establish what brain circuits control Chandelier Cells in order to further understand how they are changed in schizophrenia and other mental disorders that will hopefully lead to new and effective therapies.